The present invention relates to buffer units and fiber optic cables including the same that can meet burn, mechanical, and/or environmental requirements.
Conventional fiber optic cables comprise optical fibers which are used to transmit voice, video, and data information. The packaging of optical fibers in a fiber optic cable should facilitate installation and connectorization processes. In addition, fiber optic cables may be required to meet burn testing and general mechanical/environmental requirements.
Advances in fiber optic connector design have led to the development of multi-fiber connectors. During a typical connectorization process for conventional 900 micron tight buffered fibers with a multi-fiber connector, the bulk of buffer material at the fiber insertion side of the multi-fiber connector can result in connectorization difficulties. Additionally, handling and routing of optical fibers is difficult in fiber distribution centers and connector panels that are overcrowded with a large number of buffered fibers, subunits, and cables. Moreover, connectorization/fusion processes can cause buffer layer shrink-back that can leave optical fibers unprotected.
Mechanical and environmental tests for fiber optic cables are defined in, for example, ICEA S-83-596-1994 (ICEA-596). The mechanical tests of ICEA-596 include, for example, tensile, compression, cycle flex, and impact tests. In addition, the environmental tests of ICEA-596 include temperature cycling. Fiber optic cables not able to withstand the rigors of the foregoing tests may be rejected by customers for certain applications.
Indoor fiber optic cables have been developed for installation in plenums and risers, and/or ducts of buildings. In order for a fiber optic cable to be rated for riser or plenum use, the cable must meet flame retardance standards as determined by means of vertical or horizontal flame tests. Exemplary requirements for such tests have been established by Underwriters Laboratories (UL). Since riser cables are typically installed in vertical shafts, the relevant standard for riser rated fiber optic cables is embodied in UL 1666, a flame test in a vertical shaft without a forced air draft in the shaft. UL 1666 does not include a smoke evolution requirement. UL has promulgated the riser rating requirements in a document entitled xe2x80x9cTest for Flame Propagation Height of Electrical and Optical-Fiber Cables Installed Vertically in Shaftsxe2x80x9d, wherein values for flame propagation height are set forth. Examples of riser rated fiber optic cables are disclosed in EP-A1-0410621 and U.S. Pat. No. 5,748,823 which is incorporated by reference herein.
The relevant standard for plenum rated fiber optic cables is embodied in UL 910, a horizontal flame test setting forth flame propagation and smoke evolution requirements. In the construction of many buildings, a plenum can include, for example, a space between a drop ceiling and a structural floor above the drop ceiling. A plenum typically serves as a conduit for forced air in an air handling system, and the plenum is oftentimes a convenient location for the installation of fiber optic cables. If, in the event of a fire, the fire reaches the plenum area, flames that would otherwise rapidly propagate along non-plenum rated cables are retarded by plenum rated cables. Moreover, plenum rated cables are designed to limit smoke evolution. Riser rated cables tested to UL 1666 may not exhibit acceptable flame spread and smoke evolution results and would therefore be unsuitable for plenum use.
The UL 910 test is promulgated by UL in a document entitled: xe2x80x9cTest for Flame Propagation and Smoke-Density Values for Electrical and Optical-Fiber Cables Used in Spaces Transporting Environmental Airxe2x80x9d. A key feature of the UL 910 test is the Steiner Tunnel (horizontal forced air draft) test as modified for communications cables. During the UL 910 test, flame spread values are observed for a predetermined time (20 minutes under the current standard), and smoke is measured by a photocell in an exhaust duct. Data from the photocell measurements are used to calculate peak and average optical density values. Specifically, according to UL 910, the measured flame spread must not exceed five feet, peak smoke (optical) density must not exceed 0.5, and average smoke (optical) density must not exceed 0.15. In general, for UL 1666, the measured flame spread must not exceed 12 ft. or 850xc2x0 F.
In order to meet the foregoing burn test standards, various cable materials for the prevention, inhibition, and/or extinguishment of flame, used in riser or plenum cables, may fall into two general categories. The first category includes inherently inflammable, flame-resistant materials which are thermally stable, and may have high decomposition temperatures, for example, certain metals or high temperature plastics. The materials included in the first category can be useful as thermal/heat/flame barriers. Thermal/heat/flame barriers may have disadvantages, however, as they can be generally expensive and, because of limited burn-performance characteristics, they may be limited to a narrow range of applications.
The second general category of materials used for the prevention, inhibition, and/or extinguishment of flame includes inherently flammable materials that include flame retardant additives. Such additives actively interfere with the chemical reactions associated with combustion. Examples of inherently flammable materials are polyethylene, polypropylene, polystyrene, polyesters, polyurethanes, and epoxy resins. Typical flame retardant additives include aluminum trihydrate, metal hydroxides, brominated and chlorinated organic compounds, and phosphate compounds.
By comparison, thermal/heat/flame barriers typically do not include flame retardant additives, but are relied upon in flame protection designs for their resistance to decomposition at high temperatures, or their inherent heat dissipation properties. An example of a fiber optic cable that requires a thermal barrier, and is designed for use in plenum applications, is disclosed in U.S. Pat No. 4,941,729 which is incorporated by reference herein. The thermal barrier is a laminate of a non-flammable metallic material and a plastic material. The thermal barrier is wrapped around conductors so that longitudinally extending edges of the barrier are positioned in overlapping engagement. Although this known fiber optic cable is taught to have thermal/heat resistance, this design has several disadvantages. For example, the thermal barrier is required for flame retardance and requires two layers of material, the collective thicknesses and stiffnesses of which can result in an undesirably heavy and stiff plenum cable. The weight and stiffness can make the cable difficult to route through plenum passageways during installation, and the metal layer of the barrier typically requires grounding during installation. Additionally, the barrier necessarily contributes to manufacturing complexities and the unit cost of the cable.
It is an object of the present invention to provide a unitized fiber optic cable that facilitates connectorization, meets mechanical and environmental requirements, improves fiber packing density without increasing the cable size or flame retardant additive content, and is suitable for indoor use.
It is an object of the present invention to provide a fiber optic cable comprising: at least one buffer unit, the buffer unit comprising at least two optical fibers in a buffer layer, the buffer layer having a nominal OD of about 900 microns (xc2x1100 microns); a layer of strength members; and a cable jacket surrounding the at least one buffer unit and the strength members.
It is an object of the present invention to provide a fiber optic buffer unit comprising: optical fibers in a buffer tube having a nominal OD of about 900 microns (xc2x1100 microns), the optical fibers being tightly held by the buffer tube in a way that inhibits fiber cross-overs but permits sliding contact between the fibers and the buffer tube.
It is an object of the present invention to provide a fiber optic buffer unit comprising: optical fibers in a common buffer layer, the optical fibers being held by the buffer layer in a way that inhibits fiber cross-overs but permits sliding contact between the fibers and the buffer layer, the buffer layer comprising a composite of a liquid crystal polymer and a thermoplastic.
It is an object of the present invention to provide a fiber optic buffer unit comprising: optical fibers in a common buffer layer, the buffer layer being dimensioned such that portions of outer surfaces of each optical fiber touch each other, and other portions of the optical fibers touch the buffer layer, the optical fibers being held by the buffer layer in a way that inhibits fiber crossovers but permits sliding contact between the fibers and the buffer layer.